Multi-mode lasers having outputs with a large number of frequency or spectral components called mode lines have been utilized in the past for a variety of applications. Whether these lasers are mode-locked or not, the existence of multiple modes results in output pulses from the lasers having discrete or distinct spectra at closely spaced frequencies in the spectral region to which the laser is tuned. In the case of a mode-locked laser, the envelope of this comb-like spectral distribution is relatively smooth, and consistent as a function of time. If a laser is not mode-locked, the intensities of the various modes suffer wild fluctuations from pulse to pulse, in a random manner related to the noise statistics in the laser cavity. While in many applications production of a randomly fluctuating comb-like spectral response is useful in some applications it is desirable to both reduce the random fluctuations and to displace or shift the spectra of the output signal from the laser during the production of the output signal so that the output signal appears to have a continuous frequency spectrum within the spectral region to which the laser is tuned. In this technique the laser is first mode-locked and then the mode lines are in effect "smeared out" to give the output signal an energy vs. frequency continuum.
This combined smoothing of the envelope and smearing out of the spectra in multi-mode lasers is important in certain types of isotope separation processes in order to achieve maximum efficiency. One such process is described in U.S. Pat. No. 3,772,519 for a method and apparatus for the separation of isotopes by R. H. Levy et al. In this patent a method is disclosed for isotope separation in which an environment containing a plurality of uranium isotopes is irradiated with laser radiation of a particular frequency to selectively excite the particles of the desired isotope type. When certain particles are selectively excited, the selectively excited particles may be separated as disclosed in the patent. For optimum efficiency, the laser isotope separation process, as shown in the Levy patent, prefers excitation radiation with energy distributed throughout the band width of the absorption structure of the U-235 component of uranium vapor rather than the series of discrete mode frequencies typical of most laser radiation.
The subject invention ensures such a preferred continuous spectral distribution of energy over the bandwidth of interest, and removes the problem of severe fluctuations in the envelope of the distribution.
The subject invention also has application to atomic and molecular systems in which the exact position of the spectral components or transitions, cannot easily be either calculated or verified experimentally. Uncertainty in spectral line position may occur through Doppler broadening, Zeeman shift or cross-coupling interactions not thoroughly understood.
It is well known that mode-locking has a strong stabilizing effect on the distribution of intensity among the oscillating modes of a laser. Sweeping of the mode frequencies during the pulse to obtain effective filling of the spectral gaps on a time-integrated basis is known as "chirp". Representative methods of causing such a chirp are shown in U.S. Pat. No. 3,611,182, issued to E. B. Treacy, involving the utilization of a rotating mirror, and U.S. Pat. No. 4,088,898, issued to M. L. Stitch which utilizes a rotating optical wedge which varies in thickness and presents the varying thickness to an optical path within the resonator of the laser during the generation of a laser pulse. The latter of these two patents is assigned to the assignee of the present invention.
Additionally, broadening of the spectral response of multi-mode lasers is elaborated upon in co-pending U.S. patent application Ser. No. 862,409 filed Dec. 29, 1977 by Hans A. Bethe and Ching Sung Chang, the application also having been assigned to the assignee of this application.
Finally, chirping may be accomplished by the utilization of an electro-optic crystal in which the crystal is interposed in the laser cavity. This crystal may be a low resonant KD*P single crystal. In essence, when a voltage is applied across the crystal the index of refraction within the crystal changes. When this occurs, the effective cavity dimension is lengthened slightly. The effect of varying the cavity length during the production of a pulse, in essence, shifts the mode-lines so that a virtually continuous frequency vs. amplitude spectrum results on a time-integrated basis. It will be appreciated that the normal output characteristic of the laser is a series of spectral lines which are closely spaced with the distance between the lines being c/2L, where c is the speed of light and L is the cavity length. For lasers in general use, this spacing lies between 50 MHz and 500 MHz. It will therefore be appreciated that the shifting of the lines need not be great during the production of a pulse, in order to achieve the spectral smearing of the spectra from the laser.
The combination of mode-locking and chirp is valuable, because the mode-locking ensures a relatively uniform filling of the possible mode-structure of the laser in the band-width of interest, and then the chirping causes effective filling of the gaps between the modes. If mode-filling is absent, then the spectral envelope is very erratic. If chirp is absent, then most of the spectrum is empty because the mode-widths are much narrower than the gaps between the modes.
Since the subject invention involves mode-locking, a general discussion of mode-locking is now presented. As discussed in U.S. Pat. No. 3,935,543 issued to Ronald G. Eguchi, et al. on Jan. 27, 1976, which describes mode-locking, the frequencies of the axial modes of a laser cavity are determined by the condition that an integer number of optical half wave-lengths must fit into the cavity length L, so that the frequency difference is c/2L. Mode-locking is a process by which the axial modes of a laser cavity can be induced to oscillate with their phases "locked" together in such a way that the optical field in the laser consists of a single pulse traveling back and forth in the cavity. Aside from this spontaneous or passive locking, which can sometimes occur due to nonlinear interactions in the laser gain medium, there are two basic active methods for achieving laser mode-locking. Both require the introduction into the laser cavity of the time-varying perturbation, with the frequency of the perturbation being tuned near a value that is a multiple of the laser axial mode difference frequency. Only modes which have sufficient gain in the laser medium to overcome losses in the laser resonator will be able to oscillate. It is only these modes and perhaps some whose gain is slightly below the threshold for oscillation, that can be driven to oscillate with their phases locked together to form a sharp, optical pulse in active mode-locking schemes.
The first type of active mode-locking technique is AM locking, or loss-locking, since it involves an amplitude modulation of the optical modes. This form of mode-locking is achieved by the introduction of a time-varying loss into the laser cavity, and for simplicity, this loss could be imagined to be a very fast shutter which is being opened and closed with a frequency equal to the axial mode frequency difference. Since this frequency difference which is c/2L corresponds to the frequency at which light can traverse a round trip of the cavity, only an optical pulse timed to coincide with the time when the shutter is open can build up in the laser period. This, of course, represents a simplified picture of the loss-locking. In general, a wide variety of methods can be employed to introduce a time-varying amplitude perturbation mechanism into a laser cavity, in order to induce a locking of the phases of the modes in such a way that an optical pulse is produced that travels back and forth in the cavity to coincide with the times when the loss is minimized. For example, AM locking can be achieved by means of an electro-optic crystal. In effect, this arrangement provides for an electro-optic shutter that allows only a pulse with a pre-determined synchronization to build up in the cavity.
The other type of mode-locking is called FM locking, or phase locking. It requires modulation of the optical length of the cavity. It can be shown that such a modulation at, or close to, the frequency interval between successive modes of the laser can result in transfer of energy between these modes. It should be noted that this frequency interval can also be equated to the frequency at which light can traverse a round-trip of the cavity. Furthermore, if the modulation is strong enough, and the frequency is very closely matched to the inter-mode frequency interval, then a phase relationship develops between the modes such that the combined effect of all the modes is equivalent to a single short pulse of light circulating in the cavity; i.e., the exact duplicate result of that obtained with the previously described amplitude modulation. This FM, or phase locking, is commonly achieved by utilizing an electro-optic crystal and varying its refractive index by means of an applied electric field. An electric field applied to an electro-optic crystal produces a change in the optical index of refraction and the phase velocity of light in the medium is equal to c/n wherein c is the velocity of light in a vacuum and n is the index of refraction of the material. Thus, by means of a variable applied electric field, it is possible to modulate the phase velocity of the light in the electro-optic material. In effect, by varying the voltage applied to the crystal, the phase of the light is shifted as it propagates which is equivalent to changing the optical length of the cavity. If the voltage is applied at a frequency near a multiple of the axial mode difference frequency which is c/2L, then the typical mode-locked pulses can appear.